The invention relates to a process for the biological sewage purification of sewage containing organic as well as nitrogen-containing pollutants, in which the sewage is aerated with air and/or pure oxygen in a reactor in the presence of a biomass fixed on a support material and then, in a secondary sedimentation step, is separated into purified water and sludge. The sludge is at least partially recycled from the secondary sedimentation step back to the reactor. Also, particles of free flowing support material in lumpy and/or granulated form in the sewage are employed in the reactor as support material for the microorganisms of the biomass.
Such a process is known, for example, from EP-B 15 298, incorporated by reference herein. In this process the sewage is conducted through a reactor which is designed as a completely intermixed activated sludge tank with a circulation device or as a fluid bed or fluidized bed reactor. To attain a high purification value of particularly heavily loaded sewage, support particles of synthetic organic polymer compounds of a specific particle size are placed in the reactor. Moreover, activated sludge is, at least periodically, recycled from the secondary sedimentation step into the reactor and, in addition to the biomass fixed to the support particles, free biomass also is kept in the reactor. The support particles offer a large surface area for supporting microorganisms which, because of the macropores of the support particles, are forced to experience a decentralized growth. By this means a considerably increased material exchange surface results in comparison to the usual activated sludge flocs. The microorganisms are firmly supported in the pores of the particles of material and therefore can easily be retained in the reactor. Thus, even with very high sewage load an excellent carbonaceous BOD.sub.5 (5-day BOD) decomposition can be attained. The indicated size of the individual support particles (diameter of 5 to 50 mm, specific weight of 10 to 200 kg/m.sup.3, open macropores of 0.1 to 3 mm) results in stable bacterial settlements and guarantees an oxygen supply and a material transfer into the interior of each particle. Thus, an economical purification of BOD.sub.5 -loaded sewage is attained in an effective manner with the known process. However, with the known process only a slight decomposition of nitrogen compounds can be performed.
Form DE-OS No. 31 31 989, incorporated by reference herein, a process is known with which both a BOD.sub.5 decomposition and a nitrification, i.e., conversion of ammonia first to nitrites and then to nitrates, can be performed. For this purpose the sewage is conducted through several treatment zones one after the other and aerated there in the presence of activated sludge. In the first treatment zone or zones the decomposition of the carbon compounds is attained in a known way, and to the extent that the outflowing sewage-activated sludge mixture is loaded only slightly. Then in the last treatment zone or zones nitrification is performed in the presence of nitrifying bacteria fixed on a support material. In a subsequent secondary sedimentation step purified water is separated from sludge and removed. The sludge is partly recycled to the first treatment zone (C decomposition) and partly removed as waste sludge. In this known process there is also the possibility of feeding a part of the sewage-activated sludge mixture flowing from the nitrification zone to the first treatment zone (C decomposition) before the secondary sedimentation, to perform a denitrification of the sewage. Thus, for an extensive decomposition of the nitrogen compounds a high expenditure of energy is necessary for recycling of activated sludge from the secondary sedimentation, on the one hand, and of the sewage-activated sludge mixture from the nitrification zone to the BOD.sub.5 decomposition zone, on the other hand. Moreover, in this way a complete denitrification cannot be attained.
Moreover, from gwf (Gas and Water Engineering) 124 (1983), No. 9, pages 410-427, incorporated by reference herein, an activated sludge process with simultaneous carbon and nitrogen decomposition is known. In this article which shows the dimensioning possibility of a one-stage activated sludge installation for nitrification and denitrification, the dependence of nitrification or denitrification on sludge age (i.e., mean cell residence time), carbon respiration and sludge load is discussed. The author of the article thus comes to the following result: an optimal nitrification is attained at a BOD.sub.5 volume load of 0.5 kg/m.sub.3 .multidot.d. If a denitrification is to be performed at the same time, the tank volume is increased about 30% and a complete nitrogen separation is attained at a BOD.sub.5 volume load of 0.3 kg/m.sup.3 .multidot.d. The TKN (TKN =Total Kjeldahl nitrogen) volume load for simultaneous nitrification and denitrification in this case amounts to about 1/4 of the BOD.sub.5 volume load, in other words, to about 0.07 kg/m.sup.3 .multidot.d (TKN volume load for nitrification alone is about 0.1 kg/m.sup.3 .multidot.d).
Further, it is to be gathered from the article that the essential and limiting factor of the nitrification is the comparatively small growth rate of the nitrifying bacteria. Therefore, to attain a good nitrification performance, the sludge age must be set correspondingly high. Moreover, the entire tank volume must be sufficiently supplied with oxygen, and usually the O.sub.2 concentration is at least 2 mg/l.
In contrast with the nitrification, which represents the oxidation of NH.sub.4 nitrogen, denitrification is the use of nitrate-oxygen for respiration instead of dissolved oxygen, i.e., oxygen carried in by the aeration. A condition for denitrification is therefore that little or no dissolved oxygen be present. However, even when a small portion of dissolved oxygen is present, i.e., an amount which is no more than 1/4 of the O.sub.2 content necessary for nitrification, denitrification processes do occur, particularly in the nitrifying activated sludge tank. According to the author of the article, denitrification processes occur particularly in oxidation ditches, since dissolved oxygen is substantially no longer present at a certain distance from the aerator.
According to the author, the basic principle of simultaneous nitrification and denitrification centers around the fact that in an undivided activated sludge tank with nitrification, oxygen-free zones are created, in which the denitrification can occur. This means that the possibility of performing simultaneous nitrification and denitrification comes down to whether or not the system possesses means for appropriate aeration control. In this connection, a disadvantage of the simultaneous nitrification/denitrification step is that the resultant tank volumes necessarily become larger, since in the volume in which denitrification takes place, i.e., where there is little or no dissolved oxygen, no nitrification can take place and, in addition, the denitrification rate is reatively low. Therefore, in order to produce low nitrogen values in the discharged wastewater, a generous dimensioning of the nitrification/denitrification volume must be present.